Nanoparticulate apatite and greenalite in oldest, well-preserved hydrothermal vent precipitates

Paleoarchean jaspilites are used to track ancient ocean chemistry and photoautotrophy because they contain hematite interpreted to have formed following biological oxidation of vent-derived Fe(II) and seawater P-scavenging. However, recent studies have triggered debate about ancient seawater Fe and P deposition. Here, we report greenalite and fluorapatite (FAP) nanoparticles in the oldest, well-preserved jaspilites from the ~3.5-billion-year Dresser Formation, Pilbara Craton, Australia. We argue that both phases are vent plume particles, whereas coexisting hematite is linked to secondary oxidation. Geochemical modeling predicts that hydrothermal alteration of seafloor basalts by anoxic, sulfate-free seawater releases Fe(II) and P that simultaneously precipitate as greenalite and FAP upon venting. The formation, transport, and preservation of FAP nanoparticles indicate that seawater P concentrations were ≥1 to 2 orders of magnitude higher than in modern deepwater. We speculate that Archean seafloor vents were nanoparticle “factories” that, on prebiotic Earth, produced countless Fe(II)- and P-rich templates available for catalysis and biosynthesis.


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Figs. S1 to S8      S2).At low fluid/rock ratios, silicate hydrolysis buffers insitu pH to higher values, which promotes late-stage apatite precipitation, depending on total F concentration.This results in a decrease of total dissolved phosphate concentration from a maximum which corresponds to the highest concentration achieved before late-stage apatite precipitation occurs at lower fluid/rock ratio.At fluid/rock ratios above this, total phosphate concentration is dictated by the P content of the basalt, the mass of P-containing basalt hydrolysed, and fluid/rock ratio.

Figure S1 .
Figure S1.Equilibrium constants for the hydroxyapatite (HAP) and fluorapatite (FAP) precipitation reactions as a function of temperature at 400 and 500 bar (calculated using methods described in the main text and data by Zhu and Sverjensky (54).In general, both minerals become significantly less soluble with increases in temperature; in comparison, pressure effects are relatively minor.

Figure S3 .
Figure S3.Calculated fluid chemistry (A) and mineral mass precipitated (B) during the cooling and decompression of reaction zone fluids derived from the equilibration between SO4-free seawater and basalt/gabbro at 400 o C and 400 bar and in situ pH = 5 (corresponding to fluid composition under solubility control with FAP in Fig.S2).To simulate reaction with wall rock during fluid upflow, calculations included the reaction of fresh crystalline basalt/gabbro containing olivine with a total P concentration of 1 mol % at a water/rock mass ratio of 500.

Figure S4 .
Figure S4.Calculated concentration of total dissolve phosphate (A), in situ pH (B), and the saturation state (C) of relevant minerals during the cooling and decompression of reaction zone fluids derived from the equilibration between SO4-free seawater and basalt/gabbro containing olivine with a total P concentration of 1 mol % at 400 o C and 400 bar and in situ pH = 5 (corresponding to fluid composition in Fig.S2).To simulate reaction with wall rock during fluid upflow, calculations included the reaction of fresh crystalline basalt/gabbro containing olivine with a total P concentration of 1 mol % at a fluid/rock mass ratio of 500.

Figure S5 .
Figure S5.Effect of fluid/rock mass ratio on calculated total dissolved phosphate concentration at 250 o C after cooling and decompression of reaction zone fluids derived from the equilibration between SO4-free seawater and basalt/gabbro at 400 o C and 400 bar and in-situ pH = 5 (corresponding to fluid composition in Fig.S2).At low fluid/rock ratios, silicate hydrolysis buffers insitu pH to higher values, which promotes late-stage apatite precipitation, depending on total F concentration.This results in a decrease of total dissolved phosphate concentration from a maximum which corresponds to the highest concentration achieved before late-stage apatite precipitation occurs at lower fluid/rock ratio.At fluid/rock ratios above this, total phosphate concentration is dictated by the P content of the basalt, the mass of P-containing basalt hydrolysed, and fluid/rock ratio.

Figure S6 .
Figure S6.Calculated volume ratio between greenalite and FAP in mineral products generated in response to mixing between hydrothermal fluids and seawater.The calculations correspond to those shown in figures S3A and S4A.The calculations show that increases in the total concentration of seawater phosphate result in increases in the proportion of FAP generated relative to greenalite upon mixing.As discussed in the text, apatite reprecipitation within hydrothermal fluids ascending through the oceanic crust limit effective total P concentrations to approximately 7.5 µmol/kg under these conditions.

Figure S8 .
Figure S8.Calculated lifetime of FAP particles in seawater as a function of particle radius at 25 o C. Lifetime calculations were derived using surface-area normalised dissolution rates for FAP in modern seawater at pH 6-8 (Guidry and Mackenzie [62]) and molar volume of FAP.These constraints imply that the smallest FAP nanoparticles would dissolve in 20-75 days, which is significantly less than their estimated residence time in seawater based on Stokes law particle settling velocities.